The present invention relates to a dipole-ring magnetic circuit which can generate a magnetic field space in which magnetic field directions are aligned in one direction and the magnetic field strength is substantially uniform in an interior space of the magnetic circuit. Such a dipole-ring magnetic circuit is widely used in magnetic resonance imaging (MRI) devices, processes for manufacturing semiconductor device, and apparatuses for generating magnetic field used in basic research. Among others, it is expected that usage of the magnetic circuit will expand in the future in a magnetic-field annealing in the manufacturing processes for semiconductor device.
As shown in FIG. 4A, a dipole-ring magnetic circuit 101 has, an annular shape, and includes a plurality of permanent magnet pieces 103 arranged in a case 105 such that magnetization direction 104 of each magnet piece makes one rotation halfway around the annulus, thereby generating a magnetic field space 107 in a certain interior space in the annulus, in which magnetic field space 107 has a one-directional magnetic field direction 109 and a uniform magnetic field strength (JP2006-294851A). As shown in FIG. 4B, the magnetic circuit has a certain length in an axial direction thereof and forms a uniform magnetic field space 107 having a finite length in the axial direction in the interior space. That is, the magnetic circuit produces the uniform magnetic field space 107 of a cylindrical shape in the interior space.
The dipole-ring magnetic circuit is used in processes or devices which require a magnetic field space having a uniform magnetic field strength and a one-directional magnetic field. In recent years, it is also used in the annealing step in the manufacturing process for magneto-resistance devices (JP2004-119822A). Besides a resistive magnet type and a superconductive magnet type magnetic-field annealing, a permanent magnet type magnetic-field annealing have been increasingly used not only for producing magnetic fields with low strength, but also for producing magnetic fields with strength no less than 1 T (Tesla) in view of recent improvement in the performance of permanent magnets and power saving.
In order to improve the treating capacity of annealing, the above described uniform magnetic field space is preferably as large as possible. To increase the diameter of the magnetic field space, an inner diameter of the magnetic circuit needs to be increased. To increase the length of the magnetic field space, the length of the magnetic circuit needs to be increased. If the inner diameter is simply increased, it will cause the magnetic field strength to decline. Thus, the outer diameter of the magnetic circuit also needs to be increased to expand the magnetic field space, thereby maintaining the magnetic field strength. Accordingly, this leads to an up-scaling of size and weight of the whole.
Sintered NdFeB rare-earth magnets are predominantly used for the dipole-ring magnetic circuit because they have greater residual magnetization and greater magnetic coercive force than other magnets. A magnet will be demagnetized if a reverse magnetic field which is larger than its magnetic coercive force acts thereon. To prevent demagnetization during its use, a magnet having a magnetic coercive force greater than the reverse magnetic field acting thereon is used. However, in general, when the magnetic coercive force of a sintered NdFeB magnet is made greater, the residual magnetization of the sintered NdFeB magnet becomes lower, and vice versa. Therefore, using a magnet having an excessively high magnetic coercive force might lead to a decline of the strength of the magnetic field to be produced.
In recent years, there are reported techniques for improving a magnetic coercive force without decreasing residual magnetization by diffusing Dy (Dysprosium) or Tb (Terbium) from the surface of a sintered magnet to its interior, as seen in International Publication of WO2006/043348, and Machida K., Kawasaki T., Suzuki S., Itoh M., and Horikawa T., “Improvement of Grain Boundaries and Magnetic Properties of Nd—Fe—B Sintered Magnets (in Japanese),” Abstracts of Spring Meeting, p 202, 2004, Japan Society of Powder and Powder Metallurgy. Since such techniques can effectively concentrate Dy and Tb at grain boundaries, it is possible to increase the magnetic coercive force with almost no decrease of residual magnetization. Furthermore, since the smaller the dimension of the magnet, the deeper the added Dy or Tb diffuses into the interior, this diffusion treatment method can be applied to a small or thin magnet.
If such a treatment to diffuse Dy or Tb from the magnet surface is applied to a magnet which is not compact in size, the magnetic coercive force will not increase in the magnet interior as described in JP2010-135529A, and the magnetic coercive force will gradually increase from the interior to the surface, thus requiring attention during its use.